Book excerpt: One influencing factor on students' achievement is teachers' professional knowledge. The goal of this dissertation was to assess primary school teachers' content knowledge (CK) of physics, as one domain of professional knowledge, and analyse the impact of CK on the actual teaching and students' learning outcomes in the topic "tates of matter and phase transitions". A test assessing the teachers' CK and a video analysis scheme have been developed to describe the quality of teachers' instruction during the lessons in terms of learning process sequencing and content structure. Multilevel analyses showed a significant positive impact of teachers' CK on students' achievement depending on the way the learning opportunities were sequenced within the lesson, while no significant influence of content structure could be shown.